Dry ice is solid carbon dioxide (\(\text{CO}_2\)), maintaining an extremely low temperature of approximately \(-78.5^\circ \text{C}\) (\(-109.3^\circ \text{F}\)). It is widely used for refrigeration because it transforms directly into a gas without melting into a liquid. Using a plastic bowl is safe, but only if the container is completely open and the plastic material is appropriate. Utilizing dry ice requires careful consideration of the container’s structural integrity and the hazards associated with the large volume of gas it releases.
Material Compatibility and the Cold
The primary concern when placing dry ice in any container is the risk of thermal shock, which can compromise the material. At temperatures below \(-78.5^\circ \text{C}\), many common plastics rapidly lose flexibility, becoming brittle and prone to cracking or shattering upon impact. This structural failure occurs because the molecules within the plastic slow down and align into a more rigid arrangement. Thin or pre-cracked plastics, as well as materials like glass, are susceptible to immediate failure when exposed to this intense cold.
Plastics with good cold resistance are preferred for dry ice applications due to their retained flexibility. High-density polyethylene (HDPE) and polypropylene (PP) are materials commonly used that maintain flexibility in cold conditions. Even with these materials, it is advisable to use a thick-walled container and place a layer of insulation, such as Styrofoam or cardboard, between the dry ice and the plastic surface. This protective layer minimizes direct contact, slows the rate of heat transfer, and reduces the risk of sudden temperature change that could lead to cracking.
The Science of Sublimation and Pressure Risk
Dry ice undergoes sublimation, transitioning directly from its solid state to gaseous carbon dioxide. This phase change creates an immediate and substantial pressure hazard. The volume of gas produced is tremendous; just one pound of solid dry ice expands to generate approximately 250 liters of \(\text{CO}_2\) gas.
Any container used with dry ice must be completely open to the atmosphere or designed with a clear venting mechanism to allow the gas to escape freely. Storing dry ice in an airtight plastic container, such as a sealed jar, screw-top bottle, or a cooler without a vent, is extremely dangerous. As the gas accumulates without release, the pressure inside the sealed vessel builds rapidly, creating the potential for a violent rupture or explosion. Therefore, the plastic bowl or container should never be sealed, capped, or tightly covered while dry ice is present.
Essential Safety Practices for Handling
Beyond container selection, personal safety practices are necessary when interacting with dry ice to prevent physical harm. Direct contact with the solid material can cause severe thermal burns or frostbite in a matter of seconds. This is due to the extreme low temperature of the surface, which is far colder than a standard household freezer.
Handling dry ice should always be done with loose-fitting, insulated gloves or specialized tongs, as thin latex or nitrile gloves offer insufficient protection. Gaseous \(\text{CO}_2\) is constantly released through sublimation. Carbon dioxide gas is denser than the air we breathe, meaning it tends to sink and accumulate in low-lying areas or confined spaces. High gas concentration displaces oxygen, creating an asphyxiation risk that can cause shortness of breath or loss of consciousness, so dry ice should only be used or stored in areas with excellent ventilation.
Safe Disposal of Dry Ice
The correct way to dispose of remaining dry ice is to allow it to fully sublimate. This is done by placing the material in a well-ventilated area, such as outdoors or in a secure, open area away from foot traffic. The dry ice should be left in its original, unsealed container and allowed to dissipate into the atmosphere naturally. This method ensures the gas disperses harmlessly and leaves no liquid residue.
It is important to avoid common disposal mistakes that can lead to damage or injury. Dry ice should never be flushed down toilets or sinks. The intense cold can cause pipes to freeze, leading to cracking and potential damage to the plumbing system. Never dispose of dry ice in a sealed garbage container or trash chute, as the rapid gas buildup can easily rupture the container or cause an explosion.